The abbreviation pH stands for “potential hydrogen”: the measure of how acidic or basic (alkaline) a substance is. A scale from 1 to 14 is used to describe the amount of hydrogen ions in a liquid solution. pH 7 is defined as neutral, below 7 is acidic and above 7 is alkaline. Pure water is pH 7. For many hydroponic plants, including cannabis, the optimal pH range is between 5.5 and 6.5. Read more on Wikipedia. Each of the essential elements required for plant growth (i.e., nitrogen, potassium, phosphorus, calcium, magnesium, etc.) has its own ideal pH range for absorption by the plant roots. When the pH of the nutrient solution is too high, certain elements (e.g., iron or manganese) could be wholly or partially “locked out,” while too low a pH could cause other elements (e.g., phosphorus) to have trouble absorbing. Therefore, at too high or too low a pH, your crop could suffer from nutrient deficiencies.

You should always check the pH of the nutrient solution before feeding and before and after watering. If you use an automated hydroponic system, check the pH daily. Very good pH tri-meters are available at hydroponic stores that give you a constant pH, ppm and temperature reading of your reservoir.